1. Field of the Invention
The present invention relates to a multi-carrier radio transmission apparatus.
Satellite communication systems make use of so-called multi-carrier radio transmission technology for transmitting a plurality of channels of transmission data on respectively differing carriers. This technique has the advantage of stronger resistance to fading since the widths of the transmission bands are inversely proportional to the number of carriers. The present invention relates to a multi-carrier radio transmission apparatus in a ground station engaging in satellite communication using this technique.
2. Description of the Related Art
As will be explained later in further detail with reference to the drawings, a general multi-carrier radio transmission apparatus is, roughly speaking, comprised of a plurality of modulators and a transmitter. Each of these modulators operates using a different carrier. The carrier is modulated by the respectively corresponding transmission data D.sub.1, D.sub.2 . . . D.sub.n.
The transmitter is provided with an automatic level control (ALC) unit for performing automatic level control to maintain the transmission power at a fixed level. The automatic level control unit is comprised of an attenuator (ATT), a power detector (DET), and a comparator (COMP).
The power detector (DET) is required to be one of a high performance as it must constantly monitor the actual level of the transmission power for carrying out the automatic level control (ALC). As the power detector (DET), use is normally made of a diode. The detecting characteristic of a diode is, however, particularly nonlinear at the higher output range. The linear range is extremely narrow. That is, the dynamic range is narrow.
A diode used for a power detector (DET), however, is required to have a wide dynamic range, that is, to have an output voltage (V.sub.0) characteristic which is linear over a wide range of variation of P.sub.i.
A diode with such a wide dynamic range, however, is currently extremely expensive and use of one leads to a higher cost of the transmission apparatus.